I think that you could find a job in oil/petroleum with a BS in mech eng- getting an MS does not necessarily make you more hireable- but if you want to get an MS and then work in industry I think that again mechanical is a good choice- in industry I think that the title of your degree, ie mechanical vs petroleum might be considered, especially more so than in research- so if your petroleum plan doesn't work out you would still have a large selection of potential jobs and at the same time you can still pursue the petroleum job- but in the end I think that what matters more than any of this is who you know and what industry skills you can learn in grad school